The contribution of specific amino acids to enzymatic and pharmacological properties of a relatively toxic and a relatively non-toxic snake venom phospholipase A2 (PLA2) will be studied. Somewhat analogous studies on snake-venom neurotoxins have already provided detailed information on those amino acids essential for their biological activities. PLA2 preparations of known amino acid sequences will be isolated from H. haemachates and N. nigricollis venoms and their homogeneity determined. Derivatives of these preparations of PLA2 will be prepared which contain modifications of the following chemical groups: histidine, tyrosine, free carboxyl groups, free amino groups, arginine, tryptophan, methionine and groups modified with bifunctional reagents. The following enzymatic properties of the native and modified PLA2 preparations will be evaluated. Effects of activators and inhibitors, pH optimum, substrate specificity, effects of physical state of the substrate and hydrolysis of substrates in membranes. The following pharmacological properties of the native and modified PLA2 preparations will be studied: lethality and symptoms following intravenous and intraventricular injections; effects on electrical activity and ultrastructure of the synapses and conducting membranes of the isolated single electroplax. Pharmacological effects will be correlated with the extent of phospholipid hydrolysis in brain, junctional complexes from brain, conducting (innervated) and non-conducting (non-innervated) membranes of the isolated single electroplax. Through these studies we hope to better understand the properties of PLA2 including its active sites and those groups which are functionally or structurally essential.